To help with the sync, why not use a line cable, and output one of the stereo channels to the unused channel of the recorder handling the mono? That way you don't really even need to clap, since you'll have an exact clone of that one channel as a reference, on both decks!

This strategy works. I've done it using two DR2d recorders (4ch ea) when I needed 6 channels total but could not use a single, larger multichannel recorder. I did so by recording the headphone line-out from the first into the mic-in of the second to serve as duplicate "sync tracks". The sync tracks certainly help with alignment and sync, but keep in mind that there is a small amount of digital latency between a recorder's analog input and analog output. That means that when the sync tracks on the two recorders are perfectly aligned visually, the other tracks are not actually in perfect sync but offset by whatever the signal path latency is through the ADC>DAC of the first recorder + ADC of the second recorder. Probably only a few milliseconds which is equivalent to a few feet and thus may not be significant, but is something to be aware of.

If this worries you, you can determine perfect alignment using a clap, figure the latency offset by aligning the clap peaks, and then will be able to align perfectly using that offset the next time without needing a clap.

keep in mind that there is a small amount of digital latency between a recorder's analog input and analog output. That means that when the sync tracks on the two recorders are perfectly aligned visually, the other tracks are not actually in perfect sync but offset by whatever the signal path latency is through the ADC>DAC of the first recorder + ADC of the second recorder. Probably only a few milliseconds which is equivalent to a few feet and thus may not be significant, but is something to be aware of.

If this worries you, you can determine perfect alignment using a clap, figure the latency offset by aligning the clap peaks, and then will be able to align perfectly using that offset without needing a clap.